The invention provide a method for epitaxially growing single crystalline silicon on a silicon substrate from a silicon-bearing gas at a temperature below the pyrolytic threshold of the gas and at temperatures below those normally required for epitaxial growth.
Low temperature, e.g. 400.degree. C., deposition and epitaxial growth of single crystalline silicon is known in the art, for example Schachameyer et al U.S. Pat. Nos. 4,655,849, 4,668,304, 4,670,063, 4,670,064, 4,685,976, incorporated herein by reference. Such low temperature is significantly below typical thermal processing temperatures of 1000.degree. C. In the latter type of processing, the silicon-bearing gas is heated above its pyrolytic threshold such that the gas thermally decomposes.
The present invention uses a combination of both thermal and photolytic activation of a siliconbearing gas to epitaxially grow single crystalline silicon. In the preferred embodiment, the process is carried out at 570.degree. C. This temperature is well below the temperature normally required for epitaxial growth, and thus avoids deleterious high temperature effects on the wafer substrate. Such temperature is above the strictly photolytic range, whereby to provide some of the beneficial effects of thermal processing.
The invention also involves the use of one or more intermediate substitute bonding agents which replace the silicon-oxygen bond of an oxidized silicon wafer substrate with one or more intermediate substitute bonds which are readily processed and ultimately replaced by a silicon-silicon bond as the silicon-bearing gas is thermally and photolytically decomposed to yield atomic silicon.
In the preferred embodiment, an oxidized silicon wafer substrate is fluorinated by immersing it in a solution of dilute hydrofluoric acid. This removes the oxide layer and substitutes an adsorbed fluorinated layer for the silicon-oxygen bond. The wafer substrate is then placed in a photo-CVD (chemical vapor deposition) chamber, and the chamber is evacuated to a sub-UHV (ultra high vacuum) level of 10.sup.-3 to 10.sup.-7 Torr. Hydrogen gas is introduced into the chamber, and excimer pulsed ultraviolet laser radiation is applied through the gas and against the substrate, generally perpendicularly thereto. The radiation photolytically removes the fluorinated layer and reduces the hydrogen to atomic hydrogen such that a silicon-hydrogen bond forms in place of the fluorinated layer. During this step, the wafer substrate is optionally heated to a temperature of about 570.degree. C., which thermally aids the reaction. Disilane gas is then introduced into the chamber, and, if not already heated, the wafer substrate is heated to a temperature of about 570.degree. C. Excimer pulsed ultraviolet laser radiation is applied through the gas and against the substrate, generally perpendicularly thereto. The thermal activation by the heating and the photolytic activation by the laser radiation in combination breaks the silicon-hydrogen bond and also decomposes the disilane gas to silane and an unstable intermediate compound SiH.sub.z which decomposes to hydrogen and atomic silicon. The atomic silicon bonds with the now unbonded silicon in the substrate to epitaxially grow single crystalline silicon thereon.